This application is a renewal of the Hutchinson Study of High School Smoking (HS), a large group-randomized trial in adolescent smoking cessation that has yielded, at one year post intervention enrollment, significant treatment effects on six months prolonged abstinence at age 19. The HS trial is the first to demonstrate significant positive results for smoking cessation among adolescents proactively recruited with substantial reach in a general, non-medical setting. Addressing the great national need for adolescent smoking cessation interventions that are effective and have broad reach, the HS trial rigorously tested the effectiveness of a personally tailored smoking cessation intervention, using Motivational Interviewing (MI) and Cognitive Behavioral Skills Training (CBST), delivered via counselor-initiated telephone calls. The trial targeted all (at-least-monthly) high school senior smokers in 50 Washington high schools, recruiting a large and diverse population-based cohort that included the important and frequently neglected subgroups of less-than-daily smokers and those not thinking of quitting. The trial demonstrated that a significant majority (70%) of all adolescent smokers can be identified, contacted, and recruited proactively to a personally tailored, telephone-based smoking cessation intervention. The intervention achieved a nearly two-fold increase in 6-month prolonged cessation among daily smokers. The proposed work will build on the trial's initial positive results, and answer calls from experts in the field to learn the long-term effectiveness of, and processes underlying, teen smoking cessation intervention. We will conduct a six-year follow-up at approximately age 25 of all eligible baseline smokers (n = 2,126) to (1) determine the persistence into emerging adulthood of the trial's initial positive results on smoking abstinence and (2) among those who had not quit by age 19, determine to what extent promising results on cessation progress endpoints translate into cessation in adulthood. To better understand how the intervention impact is achieved, we will use the newly collected age 25 follow-up data together with existing data, to (3) investigate mediation of the HS intervention effect by Social Cognitive Theory-based theoretical processes, and (4) investigate the roles of MI and CBST counseling processes. The proposed investigations are critically important because they will determine the long-term impact, as measured at the end of the period of emerging adulthood, of a smoking cessation intervention delivered in late adolescence. Emerging adulthood is a period of significant smoking initiation, escalation and relapse: For nine consecutive years, emerging adults have had the highest smoking prevalence of any age group in the U.S. With cohort members now turning 25 (the end of emerging adulthood), the opportunity currently exists for conducting important and timely investigations of this initially successful adolescent intervention's potential for sustained reductions in young adult smoking prevalence.